1. Field of the Invention
This invention relates generally to perforation guns that are used in the oil and gas producing industry to explosively perforate well casing and, in particular, to an improved apparatus for releasing a perforation gun from a tubing string within a well casing after the perforation gun has been fired to perforate the well casing.
2. Description of the Background
In a typical perforation gun assembly for perforating well casing, the gun is mounted at the end of a tubing string and is positioned at the desired location within the casing by lowering the tubing string and the perforation gun down through the interior of the well casing. The gun is usually fired by detonating a cylindrically symmetrical explosive device within the perforation gun assembly. The force of the explosion perforates the casing by puncturing holes within the walls of the casing. The perforation of the casing allows oil and gas from the surrounding geological formation to flow into the interior of the casing where the oil and gas may subsequently be drawn to the surface through the tubing string.
After the perforation gun has been fired, the perforation gun assembly may either be withdrawn from the well bore or be left in the well bore permanently. When the perforation gun assembly is to be left in the well bore, the operator may desire to release the perforation gun assembly from the tubing string to remove any obstruction that it might present to the production of the oil and gas through the tubing string.
One example of a prior art release mechanism is shown in U.S. Pat. No. 4,040,482 issued to Vann on Aug. 9, 1977. In Vann, a releasable coupling from which the perforation gun is suspended is disengaged by means of mechanical dogs pivotally mounted on a releasing member that is lowered to the releasable coupling via a wireline.
Releasable perforation gun assemblies are usually attached to the tubing string via a collet finger/groove connector. In such an arrangement, the release of the perforation gun assembly is achieved by causing the collet fingers that are attached to the perforation gun assembly to move out of engagement with the complementarily shaped groove that is cut into the interior surface of one section of the tubing string. After the collet fingers have been disengaged from the groove, the perforation gun assembly falls to the bottom of the well bore where it does not interfere with the production of the oil and gas through the perforated well casing.
In prior art perforation gun assemblies that have releasable collet finger/groove connectors, the collet fingers may be moved out of engagement with the groove by slidably displacing a retaining sleeve which holds the collet fingers within the groove. Such a retaining sleeve may be displaced by a number of means including a wireline tool, a drop bar or a piston.
When a piston is used to displace the retaining sleeve, there must be a pressure differential in order to cause the piston to move. Some prior art devices utilize the pressure of the gases generated in the firing of the perforation gun to move the piston to displace the retaining sleeve and release the collet fingers. The gas pressure so generated usually must be greater than the well bore pressure. Other prior art devices are so constructed that an untimely leak in the perforation gun will cause a prematurely activated release of the release mechanism.
The present invention overcomes the problems and disadvantages that are inherent in the prior art devices. In the present invention, there is no need to generate a pressure that is greater than bottom hole pressure in order to release the perforation gun assembly from the tubing string. In addition, a premature release will not occur even if the perforation gun happens to leak.